Wireless communications systems often employ power amplifiers. An optimum power amplifier has a low level of DC power consumption and a high level of power added efficiency (i.e. ratio of the difference between the output power and the input power to DC power). In general, a power amplifier operates at maximum power efficiency when the power amplifier transmits peak output power. However, power efficiency worsens as output power decreases.
One type of a power amplifier that addresses the problem of low power efficiency at lower output powers is a Doherty amplifier. For an overview of the Doherty amplifier, see Doherty, W. H., A New High Efficiency Power Amplifier For Modulated Waves, Proceedings of the Institute of Radio Engineers, Vol. 24, No. 9, pp. 1163-82, September 1936, which is hereby incorporated by reference.
Referring to FIG. 1A, a typical Doherty amplifier 100 can include a main amplifier device 102 and an auxiliary amplifier device 104 connected in parallel. An input signal 110 can be split into a number of signals (e.g., 2) by an input splitter 112. One of the resulting signals can be coupled to an input of the main amplifier device 102 and another signal can be coupled to an input of the auxiliary amplifier device 104. Output of the main amplifier device 102 can be power combined with output of the auxiliary amplifier device 104 through a transmission line 106.
The main amplifier device 102 can be configured to be on for the entire range of output powers of the power amplifier. The auxiliary amplifier device 104 can be configured to turn on only when the main amplifier device 102 saturates. The output power at which the auxiliary amplifier device 104 turns on (and the main amplifier device 102 saturates) is referred to as a power transition point PT.
Referring to FIG. 1B, the efficiency of a typical Doherty amplifier 100 as a function of output power can be represented by a power efficiency curve 126. The power efficiency curve 126 of a typical Doherty amplifier 100 can be divided into two output power regions. In a low output power region 120, where output power is below the power transition point PT, the main amplifier device 102 is turned on and the auxiliary amplifier device 104 is turned off. In a high output power region 122, where the output power is above the power transition point PT, both the main amplifier device 102 and the auxiliary amplifier device 104 are turned on, but the main amplifier device 102 is saturated.
The power transition point PT is determined by a design parameter γ. The design parameter γ is a ratio of the maximum current through the load 108 to the maximum current delivered by the main amplifier device 102.
In the low output power region 120, as the output power increases, the power efficiency curve 126 of a typical Doherty amplifier 100 follows an upward trajectory. The power efficiency curve 126 of a typical Doherty amplifier 100 reaches a first efficiency peak 132 around a transition point PT, when the main amplifier device 102 is saturated. The power efficiency curve 126 of a typical Doherty amplifier 100 reaches a second efficiency peak 134 around a maximum output power PMAX of the typical Doherty amplifier 100, when both the main amplifier device 102 and the auxiliary amplifier device 104 are saturated.
Although a Doherty amplifier has two efficiency peaks 132, 134, the first efficiency peak 132 of a typical Doherty amplifier 100 is typically below the second efficiency peak 134. For a typical Doherty amplifier 100 with a back-off design parameter of γ, the output power delivered by the main amplifier device 102 at the second efficiency peak 134 can be 10*log 10(γ) dB higher then the power delivered by the main amplifier device 102 at the first efficiency peak 132. Several factors that can contribute to the difference between the two peaks 132, 134 follow.
The main amplifier device 102 of a typical Doherty amplifier 100 can become self biased at high power (e.g., as a result of large RF input signals 110). Self biasing can result in a large self bias induced quiescent current, and self bias induced quiescent current can bring down the overall efficiency. The amount of self bias induced quiescent current can depend on the size of the main amplifier device 102 and on the input signal 110. Accordingly, if the main amplifier device 102 has a larger size than needed to deliver the required power at the first efficiency peak 132, a large self bias induced quiescent current for the main amplifier device 102 can bring down the overall efficiency at the first efficiency peak 132.
Additionally, efficiency of a power amplifier can depend on a large signal load impedance and a large signal source impedance of the power amplifier. Large signal load and source impedances of a power amplifier can depend on the size of the power amplifier. For a smaller power amplifier, the optimal load impedance to maximize linear output power is typically larger than the optimal load impedance for a larger power amplifier. Smaller power amplifiers typically deliver lower maximum linear power into higher load impedances with high efficiency. Larger power amplifiers, on the other hand, typically deliver a higher maximum linear output power into lower load impedances with high efficiency. However, larger power amplifiers typically deliver a lower maximum linear output power into higher load impedances with poor efficiency.
Larger power amplifiers typically have larger parasitics and lower input resistance, which can reduce high frequency gain, require a different and higher Q input matching and require more reactive load impedances than smaller amplifiers. Additionally, large power amplifiers at low power levels typically operate at much lower current density, which can reduce gain and lead to linearity degradations that can directly impact efficiency.